The present invention relates to a process for the production of ethylenically unsaturated carboxylic acids or esters, particularly α, β unsaturated carboxylic acids or esters, more particularly acrylic acids or esters such as (alk)acrylic acids or alkyl (alk)acrylates particularly (meth)acrylic acid or alkyl (meth)acrylates such as methyl methacrylate by the condensation of carboxylic acid or esters with formaldehyde or a source thereof such as dimethoxymethane in the presence of catalysts, in particular, by the condensation of propionic acid or alkyl esters thereof with formaldehyde or a source thereof in the presence of such a catalyst system. The invention is therefore particularly relevant to the production of methacrylic acid (MAA) and methyl methacrylate (MMA).
As mentioned above, such unsaturated acids or esters may be made by the reaction of a carboxylic acid or ester and suitable carboxylic acids or esters are alkanoic acids (or ester) of the formula R3—CH2—COOR4, where R3 and R4 are each, independently, a suitable substituent known in the art of acrylic compounds such as hydrogen or an alkyl group, especially a lower alkyl group containing, for example, 1-4 carbon atoms. Thus, for instance, methacrylic acid or alkyl esters thereof, especially methyl methacrylate, may be made by the catalytic reaction of propionic acid, or the corresponding alkyl ester, e. g. methyl propionate, with formaldehyde as a methylene source in accordance with the reaction sequence 1.R3—CH2—COOR4+HCHO------->R3—CH(CH2OH)—COOR4andR3—CH(CH2OH)—COOR4------>R3—C(: CH2)—COOR4+H2O  Sequence 1
An example of reaction sequence 1 is reaction sequence 2CH3—CH2—COOR4+HCHO------->CH3—CH(CH2OH)—COOR4 CH3—CH(CH2OH)—COOR4------>CH3—C(:CH2)—COOR4+H2O  Sequence 2
A further reaction sequence is one which uses an acetalR3—CH2—COOR4+R′OCH2OR″------->R3—C(:CH2)—COOR4+R′OH+R″OH  Sequence 3
A theoretical example of reaction sequence 3 is reaction sequence 4 which uses dimethoxymethaneCH3—CH2—COOR4+CH3OCH2OCH3------->CH3—C(:CH2)—COOR4+2CH3OH  Sequence 4
The use of dimethoxymethane thus theoretically gives an anhydrous system which avoids the difficulty of subsequent water separation and/or subsequent product hydrolysis. In addition, the use of dimethoxymethane avoids the use of free formaldehyde but nevertheless acts in a general sense as a source of formaldehyde. The absence of water and free formaldehyde could greatly simplify the separation of MMA from the product stream.
However, in practice, Sequence 4 is problematic because methanol dehydrates to dimethyl ether and water. In addition, dimethoxymethane decomposes under catalytic conditions to dimethylether and formaldehyde. Any water formed in these reactions can hydrolyse the ester feedstock or product to its corresponding acid which may be undesirable.
U.S. Pat. No. 4,560,790 describes the production of α, β unsaturated carboxylic acids and esters by the condensation of methylal(dimethoxymethane) with a carboxylic acid or ester using a catalyst of general formula M1/M2/P/O wherein M1 is a group IIIb metal, preferably aluminium, and M2 is a group IVb metal, preferably silicon.
As mentioned above, a known production method for MMA is the catalytic conversion of methyl propionate (MEP) to MMA using formaldehyde. A known catalyst for this is a caesium catalyst on a support, for instance, silica.
U.S. Pat. No. 4,118,588 discloses the production of methyl methacrylate and methacrylic acid by reacting propionic acid or methyl propionate with dimethoxymethane in the presence of catalysts based on the phosphates and/or silicates of magnesium, calcium, aluminium, zirconium, thorium and/or titanium and also in the presence of 0 to 0.5 moles of water per mole of the acetal. The preferred phosphates are aluminium, zirconium, thorium and titanium. The catalysts generally include an oxide modifier to improve the catalytic activity. Magnesium phosphate is not exemplified and calcium phosphate is not exemplified alone but one example with an oxide modifier is provided. The results are poor compared with the other phosphates, particularly aluminium.
It is known from GB865379 that group II metal phosphates, in particular barium phosphates, can exist in crystallographic form of rhombic or cubic crystals and that these are active in catalysing the production of acrylic acid or lower alkyl esters from chloropropionic acid by direct catalytic vapour phase dehydrochlorination.